Optical fiber sensing system, sensing method, and interrogator device

Abstract

This invention provides an optical fiber sensing system, sensing method, and interrogator device that enable the installation of multiple sensor units with identical specifications, each equipped with multiple fiber optic gauges (FBGs), over long distances within an optical fiber, allowing for low-cost measurement of multiple physical and chemical quantities at numerous measurement points. [Solution] An interrogator device that sends measurement light to an optical fiber and analyzes the reflected light from the optical fiber, outputs laser light to input / output ports while continuously changing the wavelength around one of two different wavelengths by modulating the drive current of the laser light source, detects the reflected light input from the input / output ports to generate and analyze a measurement signal, discriminates the distance based on the time from when the laser light is output from the input / output ports until the reflected light is input from the input / output ports, and measures the reflected wavelength shift of the reflected light according to the discriminated distance.

Description

Technical Field

[0001] The present invention relates to an optical fiber sensing system, a sensing method, and an interrogator device.

Background Art

[0002] As a sensor technology for measuring physical quantities such as temperature and strain using an optical fiber, Patent Document 1 discloses an optical fiber sensor that uses a fiber Bragg grating formed in the core of an optical fiber as a sensor, and from the periodic change in the interference intensity between the Bragg reflected light from the sensor and the reflected light from a reference reflection end, while specifying the position of the sensor, measures the strain and temperature change of a detection unit from the amount of change in the wavelength of the Bragg reflected light from the sensor. The optical fiber sensor used in the optical frequency domain reflectometry (OFDR) method includes a plurality of sensing units for measuring strain and temperature changes consisting of fiber Bragg gratings, and an optical marking unit provided between these plurality of sensing units for specifying the position of the optical fiber that has sensed strain and temperature changes.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the technology described in Patent Document 1, fiber Bragg gratings (hereinafter referred to as FBGs) are distributed along the longitudinal direction of the optical fiber, and the location of the reflected light (signal) from each FBG is identified from the period of change in interference intensity (beat signal period). However, when the length of the optical fiber is long, some wavelengths of the input light are reflected by the FBGs that are close to the light source, causing the shape of the spectrum of the input light that reaches the FBGs that are farther from the light source to be distorted. As a result, the central wavelength of the light reflected from the FBGs farther from the light source shifts, leading to a problem of reduced measurement accuracy of distortion and temperature changes. Furthermore, in order to perform measurements using FBGs over long distances, it is necessary to reduce the distance resolution in proportion to the distance (i.e., widen the distribution spacing of each FBG), so when the FBGs are distributed in close proximity, it becomes difficult to determine which FBG the measurement signal is coming from.

[0005] On the other hand, in the generally accepted method of distinguishing each FBG by its reflection wavelength, where each FBG has a different period so that its reflection wavelength is different, the central wavelengths of the FBGs must be spaced a certain distance apart (a gap) from each other so that their reflection wavelengths do not overlap. The wavelength width (bandwidth) of the input light from the light source is, for example, 1500 to 1600 nm. If we try to arrange each central wavelength within this wavelength width with a certain distance between them, the number of central wavelengths, and thus the number of usable FBGs, will be limited to about 10 to 20. Therefore, in order to measure using a larger number of FBGs (for example, 100), it is necessary to connect multiple optical fibers, each containing about 10 to 20 FBGs, to a switch and switch between them using the switch to measure each optical fiber, which presents the challenge of high costs for laying optical fiber sensors.

[0006] The present invention has been made in view of the above points, and aims to provide an optical fiber sensing system, sensing method, and interrogator device that can install multiple sensor units of the same specifications, each equipped with multiple fiber optic gauges (FBGs), over long distances in an optical fiber, and measure multiple physical and chemical quantities at numerous measurement points at low cost. [Means for solving the problem]

[0007] The present invention includes several means for solving at least part of the above problems, but an example is as follows: an interrogator device for sending measurement light to an optical fiber and analyzing reflected light from the optical fiber, comprising a laser light source that outputs laser light, a photodetector, a processing unit, and an input / output port, wherein the processing unit controls the drive current of the laser light source to be modulated, the laser light source outputs laser light of multiple wavelengths that are different from each other, and outputs the laser light to the input / output port while continuously changing the wavelength centered on one of the different wavelengths by modulating the drive current, the photodetector detects the reflected light input from the input / output port and outputs a measurement signal, the processing unit analyzes the measurement signal and discriminates the distance based on the time from when the laser light is output from the input / output port until the reflected light is input from the input / output port, and measures the reflected wavelength shift of the reflected light according to the discriminated distance. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an optical fiber sensing system, sensing method, and interrogator device that can measure multiple physical and chemical quantities at numerous measurement points at low cost by installing multiple sensor units of the same specifications, each equipped with multiple fiber optic gauges (FBGs), over long distances within an optical fiber.

[0009] Other issues, configurations, and effects not mentioned above will be clarified by the following description of embodiments for carrying out the invention. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows an example of the configuration of the optical fiber sensing system according to the first embodiment. [Figure 2] This figure shows an example of the configuration of the optical fiber sensing system according to the second embodiment. [Figure 3] This figure shows an example of the configuration of the optical fiber sensing system according to the third embodiment. [Figure 4A] This figure shows an example of the configuration of the optical fiber sensing system according to the fourth embodiment. [Figure 4B] This figure shows an example of the configuration of the optical fiber sensing system according to the fourth embodiment. [Figure 5] This figure shows an example of the configuration of the optical fiber sensing system according to the fifth embodiment. [Figure 6] This figure shows an example of the configuration of the sensor unit in the sixth embodiment. [Figure 7A] This figure shows an example of a signal analyzed by the signal processing device in the first embodiment. [Figure 7B] This figure shows an example of a signal analyzed by the signal processing device in the first embodiment. [Figure 7C] This figure shows an example of a signal analyzed by the signal processing device in the first embodiment. [Figure 7D] This figure shows an example of a signal analyzed by the signal processing device in the first embodiment. [Figure 8A] This figure shows an example of a signal analyzed by the signal processing device in the seventh embodiment. [Figure 8B] This figure shows an example of a signal analyzed by the signal processing device in the seventh embodiment. [Figure 8C] This figure shows an example of a signal analyzed by the signal processing device in the seventh embodiment. [Modes for carrying out the invention]

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments are examples for explaining the present invention, and for the sake of clarity of explanation, appropriate omissions and simplifications have been made. The present invention can also be implemented in various other forms. Unless otherwise particularly limited, each component may be singular or plural.

[0012] In the drawings, the positions, sizes, shapes, ranges, etc. of the components shown may not represent the actual positions, sizes, shapes, ranges, etc. in order to facilitate understanding of the invention. Therefore, the present invention is not necessarily limited to the positions, sizes, shapes, ranges, etc. disclosed in the drawings.

[0013] When there are a plurality of components having the same or similar functions, they may be described by attaching different subscripts to the same reference numeral. Also, when it is not necessary to distinguish these plurality of components, the subscripts may be omitted in the description.

[0014] In the embodiments, the processes performed by executing a program may be described. Here, the computer executes the program by a processor (for example, CPU, GPU), and performs the processes defined by the program while using storage resources (for example, memory) and interface devices (for example, communication ports), etc. Therefore, the subject of the process performed by executing the program may be the processor. Similarly, the subject of the process performed by executing the program may be a controller, device, system, computer, node having a processor. The subject of the process performed by executing the program may be an arithmetic unit, and may include a dedicated circuit for performing a specific process. Here, the dedicated circuit is, for example, FPGA (Field Programmable Gate Array), ASIC (Application Specific Integrated Circuit), CPLD (Complex Programmable Logic Device), etc.

[0015] The program may be installed in a computer from a program source. The program source may be, for example, a program distribution server or a computer-readable storage medium. When the program source is a program distribution server, the program distribution server includes a processor and a storage resource for storing the program to be distributed, and the processor of the program distribution server may distribute the program to be distributed to other computers. Also, in an embodiment, two or more programs may be realized as one program, or one program may be realized as two or more programs.

Example

[0016] FIG. 1 is a diagram showing an example of the configuration of an optical fiber sensing system in the first embodiment. In FIG. 1, an optical fiber sensing system 10 includes an interrogator 100, an optical fiber 300 connected to the interrogator 100, and a plurality of sensor units (sensor units 200-1 to 200-N or a plurality of sensor units collectively referred to as sensor unit 200) connected (installed) to the optical fiber 300. The interrogator 100 inputs laser light (hereinafter referred to as laser light or light) as measurement light to the optical fiber 300 connected to an input / output port not shown, and also inputs reflected light reflected from each sensor unit 200 from the optical fiber 300. A part of the measurement light input to the optical fiber 300 is branched as incident light to the sensor unit 200 by optical couplers (optical couplers 201-1 to 201-N or all optical couplers provided on the optical fiber 300 collectively referred to as optical coupler 201) provided on the optical fiber 300 in each sensor unit 200. The branching ratio of the optical coupler 201 is set to a small value. For example, when the branching ratio of the incident light with respect to the measurement light is 1 / 1000 and 1000 sensor units 200 are arranged on the optical fiber 300, the amount of light reaching the farthest end of the optical fiber 300 is the 1000th power of 0.999, that is, about 37%, and a sufficient amount of light reaches the 1000th sensor unit 200.

[0017] Each sensor unit 200 is provided on an optical fiber branched from the optical fiber 300 by an optical coupler 201 and includes an optical coupler (optical couplers 202-1 to 202-N, or all optical couplers within each sensor unit 200 collectively referred to as optical coupler 202) that branches the incident light branched from the optical fiber 300 into at least two optical fibers, and at least two fiber Bragg gratings (FBG203-1 to FBG203-N and FBG204-1 to FBG201-N, or all FBGs within each sensor unit 200 collectively referred to as FBG203 and FBG204) provided on each of the at least two optical fibers branched by the optical coupler 202 (formed on the core of each optical fiber). Within each sensor unit 200, the incident light is branched by the optical coupler 202 and reaches FBG203 and FBG204. FBG203 and FBG204 are formed to reflect light of different wavelengths. For example, FBG203 is formed to reflect light of wavelength 1, and FBG204 is formed to reflect light of wavelength 2. As will be described later, the laser light sources (hereinafter referred to as laser light sources or lasers) 101 and 105 each emit wavelength-swept light, so the light that reaches FBG203 and FBG204 is light whose wavelength changes continuously around wavelength 1 or wavelength 2, with wavelength 1 or wavelength 2 as the central wavelength. Therefore, FBG203 and FBG204 are formed to reflect light whose wavelength changes around wavelength 1 or wavelength 2, respectively.

[0018] Furthermore, depending on the physical or chemical quantity to be measured by each sensor unit 200 or each FBG, FBG203 and FBG204 are configured in different mounting states or surface treatment states. Specifically, in the example shown in Figure 1, each FBG203 is configured to have a sensitive film attached to its surface or a sensitive material in contact with it, and this sensitive film or sensitive material is assumed to generate heat in the presence of a specific gas. For example, if the sensitive film or sensitive material is formed using titanium dioxide, the titanium dioxide, when hydrogen is present in the atmosphere, acts as a catalyst, causing a reaction between hydrogen and oxygen in the atmosphere (catalytic reaction), and generates heat depending on the hydrogen concentration. As a result, in FBG203, the grating spacing (optical path length) changes due to the change in refractive index and expansion / contraction in response to the temperature of the sensitive film or sensitive material, and the central wavelength of the reflected light changes (hereinafter, the wavelength reflected by each FBG or the central wavelength of the reflected light is called the reflected wavelength, and the change in the central wavelength of the reflected light is called the reflected wavelength shift). The amount of reflected wavelength shift changes with the temperature of the sensitive film or sensitive material. In the example above, the temperature of the sensitive film or sensitive material changes according to the hydrogen concentration. Therefore, if this amount of reflected wavelength shift can be measured by the interrogator 100, it is possible to detect the hydrogen concentration. However, since the FBG203 detects temperature changes, it is affected not only by temperature changes due to chemical reactions (catalytic reactions) as in the example above, but also by changes in ambient temperature (the ambient temperature around each sensor unit 200 and the air temperature), which degrades the accuracy of the reflected wavelength shift measurement. Therefore, another FBG204 is provided in each sensor unit 200. By not attaching a sensitive film to this FBG204 or bringing it into contact with a sensitive material, and by holding it within each sensor unit 200 without applying tension, the temperature change of the sensor unit 200 due to changes in ambient temperature, i.e., the temperature change detected by the FBG204, can be measured by the interrogator 100 as the amount of reflected wavelength shift. This allows the interrogator 100 to accurately measure the hydrogen concentration by subtracting the temperature change measured by FBG204 (another sensor unit) from the temperature change measured by FBG203.

[0019] The above is just one example; by selecting materials and substances to form the sensitive film or sensitive substance, it becomes possible to detect various substances in the air and liquids and measure their concentrations. For example, by using a sensitive substance that changes volume or generates heat upon moisture absorption, it is possible to measure temperature and humidity with the interrogator 100 from the measurement results of the reflection wavelength shift between FBG203, which has such a sensitive substance attached to or in contact with it, and FBG204, which does not have such a sensitive substance attached to or in contact with it.

[0020] Furthermore, by placing the FBG203 in close contact with the object to be measured so that a reflection wavelength shift occurs according to the strain of the object, and by measuring the temperature change with the FBG204 held without tension, it becomes possible to accurately measure the strain by subtracting the effect of the refractive index change due to temperature change from the measurement result of the FBG203.

[0021] Furthermore, if the object to be measured for strain is a diaphragm, the FGB203 can also be used as a pressure gauge for the fluid surrounding the diaphragm. For example, if a diaphragm is attached to a sealed ceramic cylinder called a tensiometer, the pressure inside the tensiometer will fluctuate depending on the moisture content in the soil, causing the diaphragm to deform. By measuring this deformation as strain on the diaphragm with the FGB203, it becomes possible to create a moisture content sensor.

[0022] Figure 1 shows an example where each sensor unit 200 has two FBGs. However, each sensor unit 200 may have three or more FBGs by using three or more optical fibers branched by the optical coupler 202 and providing an FBG to each branched optical fiber. By using three or more FBGs in each sensor unit 200 and changing the holding method (holding state), mounting state, or surface treatment state of each FBG (for example, by changing the type of sensitive film or sensitive material attached or in contact with it), it becomes possible to measure three or more types of physical or chemical quantities simultaneously with a single sensor unit 200.

[0023] Next, an example of the configuration of the interrogator 100 of the first embodiment will be described. In Figure 1, the interrogator 100 includes lasers 101 and 105, laser mounts 102 and 106, current and temperature controllers 103 and 107, a signal generator (hereinafter referred to as SG (Signal Generator)) 104, an optical switch 110, optical couplers 120a, 120b, 120c, 120d, and 120e, a circulator 121, photodetectors (hereinafter referred to as detectors) 130a and 130b, a signal processing device 150, and a control device 151.

[0024] The control device 151 controls the SG104 and the optical switch 110. The SG104 generates modulated signals for the drive current and set temperature of the lasers 101 and 105 according to the control of the control device 151, and inputs them to the current and temperature controllers 103 and 107. The modulated signal for the drive current generated by the SG104 is a swept waveform signal (e.g., a triangular wave signal). The current and temperature controllers 103 and 107 are provided corresponding to the lasers 101 and 105 (for each laser light source) and control the drive current and temperature of the lasers 101 and 105, respectively, based on the modulated signal input from the SG104. In controlling the drive current by the current and temperature controllers 103 and 107, for example, a triangular wave current (a drive current on a triangular wave) is input to the lasers 101 and 105 based on the swept waveform signal, and the drive current is modulated.

[0025] Lasers 101 and 105 are semiconductor lasers that correspond to FBGs 203 and 204, respectively, provided by the sensor unit 200, and output (oscillate) laser light of different wavelengths. For example, laser 101 oscillates laser light of wavelength 1, which is reflected by FBG 203, and laser 105 oscillates laser light of wavelength 2, which is reflected by FBG 204. Laser mounts 102 and 106 are metal blocks on which lasers 101 and 105 are mounted, respectively. Typically, temperature sensors such as Peltier elements and thermistors are incorporated inside lasers 101 and 105, or the laser mounts 102 and 106 that mount lasers 101 and 105, and the temperature control of lasers 101 and 105 is achieved by feedback control of these sensors by current and temperature controllers 103 and 107. Laser mounts 102 and 106 also play a role in dissipating heat from laser 101 and the Peltier element.

[0026] Figure 1 shows an example where the interrogator 100 is equipped with two lasers, laser mounts, and current & temperature controllers (two sets) for each of the two FBGs provided by each sensor unit 200. However, if each sensor unit 200 is equipped with three or more FBGs, the system should be configured to provide multiple lasers, laser mounts, and current & temperature controllers (multiple sets) in proportion to the number of FBGs. However, laser mounts 102 and 106 may be unnecessary depending on the type of lasers 101 and 105. In addition, instead of controlling each laser light source with a separate current & temperature controller, a single controller may be used to control the drive current and temperature of multiple lasers.

[0027] As lasers 101 and 105, for example, distributed feedback (DFB) lasers or vertical cavity surface-emitting lasers (VCSELs) are used, which are laser diodes that have a small spectral linewidth and whose oscillation frequency changes depending on the drive current. When a triangular wave drive current is input to such lasers 101 and 105 from current and temperature controllers 103 and 107 and the drive current is modulated, the oscillation frequency of lasers 101 and 105 changes continuously with time, and as a result, lasers 101 and 105 oscillate laser light that is wavelength-swept around wavelength 1 or wavelength 2, respectively. In addition to the above, to continuously change the oscillation frequencies of lasers 101 and 105, other methods may be used, for example, by using external cavity lasers (ECL) as lasers 101 and 105 and continuously changing the angle of the diffraction grating within the ECL, or by electrically controlling the cavity length of the VCSEL to continuously change the oscillation frequency.

[0028] The output light from lasers 101 and 105 is input to the optical switch 110. Based on the control signal (command) from the control device 151, the optical switch 110 alternately switches (selects) the output light from the input lasers 101 and 105 and outputs it. As a result, light with wavelength 1, emitted from laser 101, as the center wavelength, and light with wavelength 2, emitted from laser 105, are alternately output from the optical switch 110. A portion of the light output from the optical switch 110 is branched by the optical coupler 120a.

[0029] The optical coupler 102c, the two optical fibers branched by optical coupler 102c, and the optical coupler 102d that merges the two optical fibers constitute a reference interferometer. The light branched by optical coupler 120a is split into two by optical coupler 120c. The two light signals separated by optical coupler 120c pass through optical fibers of different lengths and are combined again by optical coupler 120d, causing interference (hereinafter, the interfered light is called interference light). With this configuration, the reference interferometer constitutes a Mach-Zehnder interferometer. The interference light generated (combined) by this Mach-Zehnder interferometer is detected by detector 130a and converted into an electrical signal. The reference beat signal converted into an electrical signal by detector 130a has a frequency equal to the change in laser oscillation frequency over time nΔL / c. Here, ΔL is the difference in length between the two optical fibers, n is the refractive index of the optical fiber, and c is the speed of light. This allows us to determine the time rate of change of the oscillation frequency of each laser light source. However, if laser light sources with precisely controlled time rates of change of oscillation frequency are used, a reference beat signal is not necessarily required.

[0030] After a portion of the light is branched by optical coupler 120a, the remaining light is branched again by optical coupler 120b, with a portion being input to optical coupler 120e as reference light, and the remaining light passing through circulator 121 and being input to optical fiber 300 connected to interrogator 100 as measurement light. As described above, a portion of the measurement light is branched by each optical coupler 201 to become incident light for each sensor unit 200, and reaches FBG203 and FBG204 via optical coupler 202 within each sensor unit 200. If the measurement light (incident light) has wavelength 1 as its center wavelength, FBG203 reflects that light. On the other hand, if the measurement light (incident light) has wavelength 2 as its center wavelength, FBG204 reflects that light. In this way, the reflected light reflected by FBG203 or FBG204 within each sensor unit 200 is input to interrogator 100 via optical coupler 202, optical coupler 201 and optical fiber 300. The reflected light is redirected by the circulator 121 and input to the optical coupler 120e, where it is combined with the reference light branched by the optical coupler 102b to cause interference. In this way, the optical coupler 120b, the optical fiber branched by the optical coupler 102b, the circulator 121, the optical fiber connected to the circulator 121, and the optical coupler 120e that merges the two optical fibers constitute a measuring interferometer, and as described above, it combines the reference light and the reflected light to cause interference. The interference light combined by the optical coupler 120e is detected by the detector 130b and converted into an electrical signal. The measurement signal converted into an electrical signal by the detector 130b contains a beat component with a frequency corresponding to a frequency difference of time 2nDi / c, where Di is the distance from the interrogator 100 to each sensor unit i. That is, the measurement signal is an interference signal that is the sum of the beat components from each sensor unit i.

[0031] As described above, the reference beat signal detected (converted) by detector 130a and the measurement signal detected (converted) by detector 130b are input to the signal processing device 150, converted into digital signals, and then analyzed.

[0032] In this embodiment, as shown in Figure 1, the optical path within the interrogator 100 is constructed using optical fiber components, but equivalent functions may be constructed using bulk optical components. For example, the optical switch 110 can be replaced with a switching mirror, the optical coupler 120 with a half mirror, the circulator 121 with a birefringent plate or a combination of a polarizing beam splitter, a Faraday rotator, and a half-wave plate, and the delayed optical path that forms a Mach-Zehnder interferometer sandwiched between optical couplers 120c and 120d can be replaced with a pair of mirrors.

[0033] Furthermore, as another configuration example for the interrogator 100, instead of selecting a wavelength with the optical switch 110 for measurement, it is also possible to configure it to perform measurements at two or more wavelengths simultaneously. For example, in Figure 1, instead of the optical switch 110, a WDM (Wavelength Division Multiplexing) coupler is used, which mixes light of two or more wavelengths and outputs it to a single optical fiber. Alternatively, another WDM coupler can be inserted into the two optical fibers between optical coupler 120d and detector 130a, and between optical coupler 120e and detector 130b, to separate the mixed light of two or more wavelengths based on their wavelengths, the opposite of the above. Detectors 130a and 130b are provided for each wavelength of the mixed light, and they detect the reference beat signal and measurement beat signal for each wavelength. With this configuration, the signal processing device can also perform signal analysis for each wavelength. In this configuration, not only the laser but also the detector and signal analysis processing need to be multiplexed for each wavelength. However, the ability to simultaneously detect signals from multiple types of FBG units has the advantage of enabling the detection of high-speed events such as vibrations.

[0034] As an example of a fiber optic sensing system configuration that performs measurements at two or more wavelengths simultaneously, in addition to the example above, in Figure 1, the current & temperature controller 107, laser mount 106, laser 105, and optical switch 110 are removed from the interrogator 100, and instead, multiple such interrogators are prepared, each equipped with only one current & temperature controller, laser mount, and laser. Furthermore, the lasers equipped in each interrogator are designed to emit laser light with different wavelengths (center wavelengths). A WDM coupler is inserted between the input / output ports of these multiple interrogators and the optical fiber 300, and this WDM coupler combines all the laser light (measurement light) with different center wavelengths output from each interrogator and inputs it into the optical fiber 300. The reflected light reflected back from each sensor unit 200 is separated by the WDM coupler according to its center wavelength and input to the corresponding input / output port of the interrogator. This configuration also makes it possible to simultaneously detect and measure multiple reflected light beams with different wavelengths, each reflected by a different FGB.

[0035] Next, the signal analysis in the signal processing device 150 will be explained using Figures 7A to 7D. Figures 7A to 7D show examples of signals analyzed by the signal processing device 150 in the first embodiment. As shown in Figure 1, the signal processing device 150 receives control signals (commands) input to the optical switch 110 from the control device 151. The signal processing device 150 identifies whether the light output by the optical switch 110 is light with wavelength 1 as the center wavelength emitted from the laser 101 or light with wavelength 2 as the center wavelength emitted from the laser 105, and then performs the following signal analysis processing for both types of light.

[0036] Figure 7A shows an example of the time evolution of the wavelengths of the reference light input to the optical coupler 120e and the reflected light from each sensor unit 200. As described above, since the laser 101 or laser 105 oscillates wavelength-swept light, the wavelength of the reference light, as shown in Figure 7A, starts to change from the time when the light from the laser 101 or laser 105 starts oscillating (hereinafter referred to as the reference time) and changes continuously over time. On the other hand, the reflected light from each sensor unit 200, whose distance from the interrogator 100 is Di, starts to change in wavelength with a delay of 2nDi / c from the reference time, as exemplified by the reflected light from sensor unit 1 and sensor unit N in Figure 7A, and, like the reference light, its wavelength changes continuously.

[0037] Figure 7B shows an example of a reference beat signal detected by detector 130a. As shown in Figure 7B, the reference beat signal is a beat signal with an amplitude proportional to the intensity of laser 101 or laser 105 (light source). Note that depending on the type and control method of lasers 101 and 105, the rate of change of the wavelength is not constant, in which case the beat frequency fluctuates within the wavelength sweep period (modulation period of the drive current of lasers 101 and 105). In that case, the reference beat signal detected by detector 130a is binarized, or the signal obtained by multiplying and then binarizing it is used as a sampling pulse to convert the measurement signal into a digital signal. Alternatively, the reference beat signal is converted into a digital signal, and then the phase of the reference beat signal is analyzed by the signal processing device 150, and the measurement signal is resampled at timings of a constant phase interval of the reference beat signal. By performing such processing, the period of the beat component of the measurement signal is stabilized.

[0038] Figure 7C shows an example of a beat signal originating from one of the sensor units i of the measurement signal detected by the detector 130b. The beat signal originating from one sensor unit i is a signal whose amplitude changes by multiplying the amplitude of the reference beat signal by the wavelength characteristics of the reflectivity of the FBG. By extracting only the beat component corresponding to the frequency difference of time 2nDi / c from the measurement signal detected by the detector 130b (after the above resampling process has been performed as necessary) using a band-pass filter, a beat signal like the one shown in Figure 7C can be obtained. Band-pass filters for various beat frequencies can be implemented by digital signal processing in the signal processing device 150.

[0039] Figure 7D shows an example of the results of amplitude detection processing for beat signals originating from each sensor unit 200. When amplitude detection processing is performed using a band-transmittance filter on beat frequencies corresponding to the distance Di from the interrogator 100 to each sensor unit 200, data (reflection intensity spectrum) is obtained, as shown in Figure 7D, with wavelength on the horizontal axis, distance on the vertical axis, and brightness / darkness as amplitude. Amplitude detection processing can be achieved, for example, by finding the square root of the sum of the squares of each beat signal originating from each sensor unit 200 and the signal obtained by applying a Hilbert transform to it. By obtaining the data shown in Figure 7D, the signal processing device 150 can determine the position of the amplitude peak for each distance (i.e., each sensor unit). Furthermore, by performing the same processing as described above on each reflected light when measurement light is reflected off FGB203 and FGB204 within each sensor unit 200, under conditions where there is no temperature change and no tension is applied (i.e., FGB203 and FGB204 remain completely unchanged), the signal processing device 150 obtains data similar to that shown in Figure 7D in advance and determines the amplitude peak position (reference peak position) for each sensor unit 200. The signal processing device 150 can determine the reflection wavelength shift of FBG203 and FBG204 of each sensor unit 200 by calculating the difference between the amplitude peak position obtained from the data shown in Figure 7D and the reference peak position for each sensor unit 200.

[0040] Figures 7C and 7D are schematic diagrams showing the case where the wavelength sweep range that can be changed by modulating the drive current for a single laser covers the reflection wavelengths (wavelength 1 and wavelength 2) of the two FBGs 203 and 204 within each sensor unit 200 at once (when the laser can oscillate light of two reflection wavelengths by modulating the drive current). However, in the interrogator 100 of the first embodiment shown in Figure 1, lasers 101 and 105 each oscillate light of only one wavelength. In this case, Figure 7C will show only the beat signal of either the left or right side, and Figure 7D will also show only the data of either wavelength 1 or wavelength 2.

[0041] In the above description of the interrogator 100, the signal processing device 150 and the control device 151 were described as separate devices. However, it is also possible to configure the interrogator 100 with a single computer or processing device (processor) that performs both functions (i.e., signal analysis and control of SG104, etc.). In this case, the interrogator 100 includes the computer or processing device (processor) and performs signal analysis and various controls. The same applies to other embodiments described later.

[0042] As explained above, the optical fiber sensing system in the first embodiment allows for the arrangement of numerous sensor units regardless of the length (distance) of the optical fiber, by connecting the sensor units via optical couplers provided on the optical fiber. In this regard, if, as in the conventional technology, numerous sensor units equipped with FBGs are directly arranged on the optical fiber and an attempt is made to distinguish each FBG included in these numerous sensor units by the difference in reflection wavelength, then, considering the margin for change in reflection wavelength due to distortion and refractive index change, if the center wavelengths of each reflection wavelength are arranged so that their reflection wavelengths do not overlap, even if the wavelength width of the input light from the light source is wide, only about 20 to 30 sensor units can be arranged on the optical fiber at most. For example, if the wavelength width of the laser light emitted by the laser light source is about 90 nm, the wavelength band is 1550 nm, and the temperature change width of the sensor unit is 100°C or the strain change width is 600 ppm, then the change in reflection wavelength will be 1 nm. If we allow an additional 1nm margin on both sides of the 1nm reflection wavelength change of each FBG, approximately the width of the reflection wavelength spectrum, the wavelength width per FBG becomes 3nm. Dividing this by the laser light wavelength of 90nm gives us a maximum of 30 placement points. Furthermore, this would require manufacturing each sensor separately, increasing costs.

[0043] In contrast, as described above, in the first embodiment, each sensor unit is connected to the end of an optical fiber that has been branched by an optical coupler attached to the optical fiber at a certain interval or longer. Since there are differences in the distance from the interrogator at the connection positions of each sensor unit, if an interrogator is used that can discriminate between distance differences and measure the spectrum of the reflected wavelength for each distance, it becomes possible to discriminate between the reflected wavelengths from each sensor unit. Furthermore, when using long-distance optical fibers, attempting to discriminate between each sensor unit connected to the optical fiber based on distance differences reduces the distance resolution, i.e., the discriminative ability. However, by making the installation interval (interval between connection positions) of each sensor unit longer than the distance resolution, the interrogator can discriminate between each sensor unit.

[0044] Furthermore, since multiple FBGs within each sensor unit are discriminated based on differences in reflection wavelength, the interrogator can distinguish between FBGs even if the distance difference between them is smaller than the interrogator's distance resolution, or even if there is no distance difference. This eliminates the need to maintain distance differences between multiple FBGs within each sensor unit, enabling the construction of a compact sensor unit. Moreover, by connecting numerous sensor units of the same specifications configured in this way and discriminating between them based on distance differences, a low-cost optical fiber sensing system can be realized. In addition, by placing multiple FBGs within each sensor unit in different holding or mounting states (attachment or contact of sensitive film or sensitive material, etc.), it becomes possible to measure multiple physical and chemical quantities simultaneously. For example, by attaching a gas-sensitive film to only one of two types of FBGs, the influence of temperature changes measured by the other FBG can be eliminated, enabling highly sensitive gas detection. [Examples]

[0045] In the first embodiment, an example was described in which the interrogator is equipped with multiple sets of lasers, laser mounts, and current & temperature controllers according to the number of FBGs in each sensor unit, and light of the wavelength reflected by each FBG is emitted from the laser corresponding to each FBG. However, it is also possible to configure the system to emit light of multiple wavelengths from a single laser. In the second embodiment, an example of an optical fiber sensing system (specifically an interrogator) equipped with such a configuration will be described. Note that in the following description, explanations that overlap with the first embodiment will be omitted, and only the differences will be described.

[0046] Figure 2 shows an example of the configuration of the optical fiber sensing system in the second embodiment, and the same reference numerals are used for components identical to those in the optical fiber sensing system 10 shown in Figure 1. In Figure 2, the optical fiber sensing system 20 includes an interrogator 160, an optical fiber 300 connected to the interrogator 160, and a plurality of sensor units 200 connected to the optical fiber 300, similar to the optical fiber sensing system 10 shown in Figure 1. Unlike the interrogator 100 shown in Figure 1, the interrogator 160 includes only a laser 161, a laser mount 102, and a current & temperature controller 103, and instead of an optical switch 110, it includes a modulator / optical amplifier 111. The other components of the interrogator 160 are the same as those of the interrogator 100 shown in Figure 1, and the configuration of each sensor unit 200 is also the same as that shown in Figure 1.

[0047] In the second embodiment, a DFB laser or VCSEL is used as the laser 161, which can change the oscillation wavelength depending on the temperature of the laser element, and the oscillation wavelength of the laser 161 is switched by switching the set temperature of the laser 161. Specifically, the current and temperature controller 103 controls the set temperature of the laser 161 element to first oscillate light of wavelength 1, and modulates the drive current of the laser 161 in the same manner as in the first embodiment to cause the laser 161 to oscillate light swept around wavelength 1. Next, the set temperature of the laser 161 element is controlled to oscillate light of wavelength 2, and modulates the drive current of the laser 161 to cause the laser 161 to oscillate light swept around wavelength 2. With this configuration, if the wavelength sweep range that can be changed by modulating the drive current for the laser 161 cannot cover the reflection wavelengths (wavelength 1, wavelength 2) of multiple FBGs 203 and 204 (i.e., if the laser 161 cannot oscillate light of two wavelengths simply by changing the drive current), it becomes possible to make the laser 161 oscillate light of multiple wavelengths by switching the set temperature for the laser 161.

[0048] As described above, the light emitted from laser 101 is input to the newly installed modulator / optical amplifier 111. When the wavelength sweep of laser 161 is performed by modulating the drive current, the emission intensity of laser 161 also changes along with the wavelength. Therefore, a modulation signal for the modulator / optical amplifier 111 is generated by SG104 along with the modulation signal for the drive current and input to the modulator / optical amplifier 111. Specifically, generally, when the drive current of a semiconductor laser is gradually increased, the wavelength of the emitted light gradually shifts to longer wavelengths, and at the same time, the emission intensity also gradually increases. Therefore, SG104 generates a modulation signal such that the gain of the modulator / optical amplifier 111 gradually decreases and inputs it to the modulator / optical amplifier 111. This reduces the change in the intensity of the output light from the modulator / optical amplifier 111 that occurs due to the change in the emission intensity of laser 161 caused by the modulation of the drive current. Furthermore, if the drive current is gradually reduced and the wavelength of the oscillating light is gradually shifted to a shorter wavelength, a modulation signal with the opposite phase to the above can be generated by SG104 and input to the modulator / optical amplifier 111.

[0049] As described above, since the modulator / optical amplifier 111 can suppress changes in the emission intensity of the output light from the semiconductor laser, if the modulator / optical amplifier 111 is inserted before and after the optical switch 110 in the interrogator 100 of the first embodiment and the same control is performed, it is possible to similarly suppress changes in the emission intensity of the output light from the lasers 101 and 105.

[0050] As described above, the light emitted (output) from the laser 161 and whose emission intensity is suppressed by the modulator / optical amplifier 111 is input to the optical fiber 300 as measurement light from the interrogator 160, similar to the first embodiment. This light becomes the incident light for each sensor unit 200, is reflected by FBG203 or FBG204 according to its wavelength, and is input back to the interrogator 160 as reflected light, detected as a measurement signal by the detector 130b, analyzed by the signal processing device 150, and the reflected wavelength shift of FBG203 or FBG204 in each sensor unit 200 is measured. At this time, similar to the first embodiment, the signal processing device 150 of the interrogator 160 can distinguish each sensor unit 200 based on the difference in distance.

[0051] In the description of the second embodiment above, we described the case where each sensor unit 200 has two FBGs, with reflection wavelengths of wavelength 1 and wavelength 2 respectively. However, even when each sensor unit 200 has three or more FBGs, and the reflection wavelengths of each FBG are different from each other, the reflection wavelength shift of each FBG can be measured by switching the set temperature of the laser 161 for each of those wavelengths to cause light to oscillate.

[0052] As explained above, the optical fiber sensing system in the second embodiment offers the same advantages as the first embodiment, but because the interrogator only needs to consist of one set of laser, laser mount, and current & temperature controller, the interrogator can also be configured at a low cost and compactly, enabling further cost reduction and compactness of the entire optical fiber sensing system. [Examples]

[0053] In the first and second embodiments, it was explained that even when using long-distance optical fibers, the signal processing device of the interrogator can distinguish each sensor unit by setting the installation interval (connection interval) of each sensor unit to be longer than the distance resolution. However, when using optical fibers longer than the coherence distance of the laser, interference between reflected light and reference light becomes less likely for sensor units connected at positions farther than the coherence distance of the laser, making discrimination and measurement of reflected wavelength shift difficult for such sensor units. Therefore, in the third embodiment, an example configuration of an optical fiber sensing system (specifically an interrogator) that enables discrimination and measurement of reflected wavelength shift for sensor units installed at positions farther than the distance limited by the coherence distance of the laser will be described. Note that in the following description, explanations that overlap with the first and second embodiments will be omitted, and only the differences will be described.

[0054] Figure 3 shows an example of the configuration of the optical fiber sensing system in the third embodiment, and the same reference numerals are used for components identical to those of the optical fiber sensing systems 10 and 20 shown in Figures 1 and 2. In Figure 3, the optical fiber sensing system 30 includes an interrogator 170, an optical fiber 300 connected to the interrogator 170, and a plurality of sensor units 200 connected to the optical fiber 300, similar to the optical fiber sensing systems 10 and 20 shown in Figures 1 and 2. The interrogator 170 has the same configuration as the interrogator 160 shown in Figure 2, and further includes a reference optical path length switch 140 provided between the optical coupler 120b and the optical coupler 120e.

[0055] The reference optical path length switch 140 includes a circulator 141, an optical switch 142, multiple reference optical fibers of different lengths (optical path lengths) (optical fibers 143-1 to 143-K (where K is the number of optical fibers in the reference optical path length switch 140), or all reference optical fibers collectively referred to as optical fiber 143), and mirrors 144 provided at the end of each optical fiber 143. The required number of optical fibers is determined by dividing the maximum distance of the optical fiber 300 on which the sensor unit 200 is installed (the distance to the furthest sensor unit 200-N) by the coherence distance of the laser 161. For example, if the maximum distance is 1000m and the coherence distance is 100m, the reference optical path length switch 140 needs to have at least 10 optical fibers. Furthermore, each optical fiber 143-1 to 143-K is adjusted to a length (optical path length) corresponding to its respective distance (for example, in the above case, optical fiber 143-1 is 100m, 143-10 is 1000m, etc.).

[0056] Similar to the first and second embodiments, the light output from the laser 161, passing through the modulator / optical amplifier 111 and optical coupler 120a, is branched by optical coupler 120b, and a portion is input to the reference optical path length switch 140 as reference light. Within the reference optical path length switch 140, the reference light is redirected by the circulator 141 and input to the optical switch 142. The optical switch 142 switches the output destination to an optical fiber 143 corresponding to the installation distance of the sensor unit 200 to be measured (for example, in the above case, if the installation distance of the sensor unit 200 to be measured is 100m or less, optical fiber 143-1; if the installation distance of the sensor unit 200 to be measured exceeds 900m, optical fiber 143-10), and outputs the reference light. The reference light travels through the output optical fiber 143, is reflected by the mirror 144, and returns to the optical switch 142. The reference light is output from the optical switch 142, redirected again by the circulator 141 towards the optical coupler 120e, output from the reference optical path length switch 140, and input to the optical coupler 120e. The reference light is combined with the reflected light at the optical coupler 120e, causing interference. By configuring the reference optical path length switch 140 in this way, the optical switch 142 switches the output destination of the reference light to an optical fiber 143 corresponding to the sensor unit 200 to be measured, that is, by switching to an optical fiber 143 with an optical path length corresponding to the distance to the sensor unit 200 to be measured, the reference light that has traveled back and forth through that optical fiber 143 will have a phase delay of about the same amount as the reflected light from the sensor unit 200 to be measured, making it easier to interfere with the reflected light from the sensor unit 200 to be measured. Therefore, even if the distance to the installation position of the sensor unit 200 to be measured is greater than the coherence distance of the laser 161, it becomes possible to easily interfere the reference light and the reflected light, and it becomes possible to measure the reflected wavelength shift.

[0057] As explained above, the optical fiber sensing system in the third embodiment offers the same advantages as the first and second embodiments. In addition, because the interrogator is equipped with a reference optical path length switch, it becomes possible to easily interfere the reference light and the reflected light, even when the sensor to be measured is located at a position farther than the coherence distance of the laser, thereby enabling the measurement of the reflected wavelength shift. [Examples]

[0058] In the first and second embodiments, a measurement signal is generated using interference light obtained by combining reference light and reflected light. However, using interference light limits the distance at which reflected light can be measured due to the coherence distance of the laser. Therefore, in the third embodiment, an example of an interrogator configuration that enables measurement of the reflected wavelength shift for a sensor unit installed at a position farther than the coherence distance of the laser was described. In contrast, the fourth embodiment describes an example of an optical fiber sensing system (specifically, an interrogator) configuration that extends the distance at which the interrogator can measure (the distance to the installation position of the sensor unit) by not using interference light. Note that in the following description, explanations that overlap with the first and second embodiments will be omitted, and only the differences will be described.

[0059] Figures 4A and 4B show an example of the configuration of the optical fiber sensing system in the fourth embodiment, and the same reference numerals are used for components identical to those of the optical fiber sensing systems 10 and 20 shown in Figures 1 and 2. In Figures 4A and 4B, the optical fiber sensing system 40 includes an interrogator 180, an optical fiber 300 connected to the interrogator 180, and a plurality of sensor units 200 connected to the optical fiber 300, similar to the optical fiber sensing systems 10 and 20 shown in Figures 1 and 2. The interrogator 180 includes a laser 161, a laser mount 102, a current and temperature controller 103, an SG 104, a modulator / optical amplifier 111, a circulator 121, a detector 130b, a signal processing device 150, and a control device 151, similar to the interrogator 160 shown in Figure 2, but does not include the other optical couplers 120a to 120e and the detector 130a. As a result, the light output from the modulator / optical amplifier 111 passes through the circulator 121 without being branched and is input to the optical fiber as measurement light. The reflected light from each sensor unit 200 is combined with the reference light and input directly to the detector 130b via the circulator without interference. The detector 130b directly converts the reflected light into photoelectric energy to detect the measurement signal.

[0060] In the fourth embodiment, the SG104 generates a modulation signal 400 that is different from the modulation signal of the drive current in the first and second embodiments and inputs it to the current and temperature controller 103 or the modulator / optical amplifier 111. As an example of the modulation signal 400, Figure 4A shows an example in which the SG104 generates and outputs a signal that is pulsed and switches on / off for a short time (hereinafter referred to as a pulse signal). In this way, the modulation signal (pulse signal) 400 is generated by the SG104 and input to the current and temperature controller 103, for example, and the drive current is controlled in the current and temperature controller 103 based on the modulation signal (pulse signal) 400, so that pulsed light is emitted from the laser 161. Also, when the modulation signal (pulse signal) 400 is input to the modulator / optical amplifier 111, the modulator / optical amplifier 111 outputs light at the on / off timing of the modulation signal (pulse signal) 400, so that pulsed light is output from the modulator / optical amplifier 111. The pulsed light output in this manner enters each sensor unit 200 from the optical fiber 300, is reflected by one of the FBGs of each sensor unit 200, and this reflected light is input to the detector 130b via the circulator 121, where it is detected as a measurement signal.

[0061] As explained with respect to Figure 7A, in the detected measurement signal, the pulse is delayed by a time (2nDi / c) corresponding to the distance from the interrogator 180 to each sensor unit 200 relative to the modulated signal (pulse signal) 400. The reflectance of each sensor unit 200 relative to the oscillation wavelength of the laser 161 appears as the amount of detected light at a position delayed by a time corresponding to the distance to each sensor unit 200. Therefore, by slowly changing the oscillation wavelength of the laser 161 relative to the period of the modulated signal (pulse signal) 400, reflectance data of each sensor unit 200 for each oscillation wavelength can be obtained. That is, data as shown in Figure 7D can be obtained, and the reflected light from each sensor unit 200 can be discriminated by the delay time of the measurement signal relative to the modulated signal (pulse signal) 400. Note that in Figure 7D the vertical axis was distance, but in this case the vertical axis of the data corresponds to the pulse delay amount (the time mentioned above).

[0062] In addition to the pulse signal described above, it is also possible to use a signal that randomly switches on and off over time (hereinafter referred to as a pseudo-random number signal) as the modulation signal. Figure 4B shows an example in which SG104 generates and outputs such a pseudo-random number signal as the modulation signal 400. A known pseudo-random number sequence is called an M-sequence. This sequence has the property that when the random number sequence is shifted and the correlation is taken, the correlation value is not zero only when the shift amount is zero, and the correlation value is zero for all other shift amounts. Therefore, the signal processing device 150 calculates the correlation value between the measurement signal and the modulation signal (pseudo-random number signal) 400 output from SG104 with respect to the measurement signal detected by the detector 130b, while delaying (time-shifting) the modulation signal (pseudo-random number signal) 400 output from SG104. The correlation value calculated when the signal is shifted by a time equivalent to the distance to each sensor unit 200 (by the time shift amount equivalent to the distance) is obtained as the reflectance of each sensor unit 200 with respect to the oscillation wavelength of the laser 161. Similarly, by slowly changing the oscillation wavelength of the laser 161 relative to the period of the modulation signal (pseudo-random number signal) 400, reflectance data of each sensor unit 200 for each oscillation wavelength can be obtained. That is, data like that shown in Figure 7D can be obtained. In this case, the vertical axis of the data corresponds to the amount of time shift when calculating the correlation value.

[0063] Whether the modulated signal 400 is a pulse signal or a pseudo-random number signal, by obtaining the above data, the signal processing device 150 can determine the peak position of the reflectance for each sensor unit 200, similar to the first embodiment, and from the determined peak position, it can determine the reflection wavelength shift of FBG203 and FBG204 of each sensor unit 200.

[0064] As explained above, the optical fiber sensing system in the fourth embodiment offers the same advantages as the first and second embodiments, but by using pulse signals or pseudo-random number signals as the modulation signals generated by the SG, it becomes possible to extend the distance that the interrogator can measure (the distance to the installation location of the sensor unit). [Examples]

[0065] In the fifth embodiment, similar to the fourth embodiment, another configuration example of an optical fiber sensing system (specifically, an interrogator) is described that extends the distance that the interrogator can measure by not using interfering light. In the following description, explanations that overlap with the first, second, and fourth embodiments are omitted, and only the differences are described.

[0066] Figure 5 shows an example of the configuration of the optical fiber sensing system in the fifth embodiment, and the same reference numerals are used for components identical to those of the optical fiber sensing systems 10, 20, and 40 shown in Figures 1, 2, and 4. In Figure 5, the optical fiber sensing system 50, like the optical fiber sensing system 40 shown in Figure 4, includes an interrogator 190, an optical fiber 300 connected to the interrogator 190, and a plurality of sensor units 200 connected to the optical fiber 300. The interrogator 190 has the same configuration as the interrogator 180 shown in Figure 4, and further includes a frequency mixer (hereinafter referred to as frequency mixer) 152 and a low-pass filter (LPF) 153 provided between the detector 130b and the signal processing device 150. The frequency mixer 152 mixes (combines) the measurement signal detected by the detector 130b and the modulated signal 400 output from the SG 104 to generate a beat signal. This beat signal contains a mixture of the measurement signal detected by the detector 130b and the sum frequency beat signal and difference frequency beat signal of the modulation signal 400 output from the SG104. Therefore, the low-pass filter 153 extracts only the beat component corresponding to the frequency difference between the measurement signal and the modulation signal 400 from the beat signal generated by the frequency mixer 152 and outputs it to the signal processing device 150.

[0067] In this embodiment, a chirp signal whose frequency changes linearly with time is used as the modulation signal 400, and, as in the fourth embodiment, the SG104 generates and outputs such a chirp signal as the modulation signal 400. As described above, the measurement signal and the modulation signal (chirp signal) 400 are mixed in the frequency mixer 152, generating a beat signal with a frequency proportional to the distance to each sensor unit 200. This is similar to how the beat signal is generated by the optical coupler 120e and the detector 130b in the first embodiment. The difference is that in the example described in the first embodiment, light with a continuously changing oscillation frequency of the electric field of light, i.e., wavelength, is output, and the beat signal is generated by optical interference between the measurement light and the reflected light of the electric field signal of light, whereas in the example of this embodiment, the intensity of light is modulated with a chirp signal, the reflected light returning from each sensor unit is converted into an electrical signal by the detector 130b, and then the beat signal is generated by electrical wavelength mixing with the original chirp signal. Furthermore, the signal analysis processing performed by the signal processing device 150 after the beat signal is generated is the same as that described in the first embodiment.

[0068] As explained above, the optical fiber sensing system in the fifth embodiment offers the same advantages as the first and second embodiments, and, similar to the fourth embodiment, it also allows for extending the distance that the interrogator can measure (the distance to the installation location of the sensor unit). [Examples]

[0069] In the sixth embodiment, other configuration examples of the sensor unit described in detail in the first embodiment will be explained. In the following description, explanations that overlap with the first embodiment will be omitted, and only the different parts will be explained. Figure 6 is a diagram showing an example of the configuration of the sensor unit in the sixth embodiment, and the same reference numerals are used for components that are the same as those in the sensor unit shown in Figure 1. In Figure 6, sensor unit 200-N is shown as a representative of the sensor unit 200, but the same applies to other sensor units 200.

[0070] In Figure 6, each sensor unit 200 includes FBG203 and FBG204 formed in series on a single optical fiber. The optical fiber on which each FBG is formed is connected to an optical coupler 201 provided on the optical fiber 300, and the light branched from the optical fiber 300 to the sensor unit 200 at the optical coupler 201 reaches FBG203 and 204. FBG203 and FBG204 are the same as in the first embodiment and are formed to reflect light of different wavelengths. Therefore, the reflected light from the two FBGs can be discriminated by their reflection wavelengths, making it possible to measure the reflection wavelength shift of each FBG without them influencing each other. Although Figure 6 shows an example where two FBGs are formed in series, three or more FBGs with different reflection wavelengths may be formed in series. Furthermore, by placing these FBGs in different holding and mounting states, it becomes possible to measure multiple physical and chemical quantities.

[0071] As explained above, with the sensor unit of the sixth embodiment, even when multiple FBGs are formed in series on a single optical fiber within the sensor unit, the same effects as in the first embodiment can be obtained. [Examples]

[0072] In the seventh embodiment, another example of signal analysis by the signal processing device 150, which was described in detail in the first embodiment, will be explained. In the following description, explanations that overlap with the first embodiment will be omitted, and only the differences will be explained. Figures 8A to 8C are diagrams showing examples of signals analyzed by the signal processing device 150 in the seventh embodiment. In the signal analysis example described in the first embodiment, using the reference beat signal shown in Figure 7B, a beat signal as shown in Figure 7C is obtained by using a bandpass filter or by dividing the data into time intervals and performing a Fast Fourier Transform (FFT) on the measurement signal detected by the detector 130b, and extracting beat components corresponding to frequencies corresponding to the distance of each sensor unit 200. In contrast, in this embodiment, the signal processing device 150 performs an FFT on the entire measurement signal detected by the detector 130b.

[0073] Figure 8A shows an example of the result of performing an FFT on the entire measurement signal detected by the detector 130b. By performing an FFT on the entire measurement signal, spectral peaks appear symmetrically at the positive and negative positions of the beat frequency corresponding to the distance of each sensor unit 200, as shown in Figure 8A. The signal processing device 150 extracts only one of the spectral peaks corresponding to the distance of the sensor unit 200 being measured and obtains a complex spectrum shifted near the frequency origin. Figure 8B shows an example of the complex spectrum obtained in this way. The signal processing device 150 performs an inverse FFT on this complex spectrum and calculates its absolute value. Figure 8C shows an example of the result of performing an inverse FFT on the complex spectrum and calculating its absolute value. As a result of such calculations, as shown in Figure 8C, data on the change in amplitude of the extracted spectral peak (beat frequency component) over time is obtained. The horizontal axis of this data is time, but since the wavelengths of lasers 101 and 105 are changed over time, it corresponds to the wavelength. Therefore, Figure 8C shows the reflected wavelength spectrum of the sensor unit 200 being measured. As shown in Figure 8A, by repeating the same process for each spectral peak corresponding to the distance of each sensor unit 200, the reflection wavelength spectrum of all sensor units 200 can be obtained.

[0074] Although the inverse FFT needs to be performed for each sensor unit 200, this inverse FFT only needs to be performed on a limited number of complex spectra, including the width of each spectral peak. Since the FFT / inverse FFT can be performed in a computation time proportional to MlogM for the number of data points M, if the width of the data extracted for the inverse FFT is m, then the sum of the computation times for the inverse FFT with respect to the number of peaks N is proportional to N·mlogm. In this case, N·m is smaller than the number of data points (M) of the original FFT. Therefore, the sum of the computation times for the inverse FFT (proportional to N·mlogm) is shorter than the computation time required for the original FFT (proportional to MlogM). In this way, the reflection wavelength spectra of all sensor units can be obtained through high-speed processing, and the reflection wavelength shift of each FBG of each sensor unit 200 can be determined more quickly.

[0075] As explained above, according to the signal analysis example in the seventh embodiment, it is possible to determine the reflection wavelength shift of each FBG in each sensor unit at the same speed as or faster than the signal analysis example in the first embodiment.

[0076] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various design modifications can be made without departing from the spirit of the invention as described in the claims. For example, each of the embodiments described above has been described in detail in order to explain the present invention in an easy-to-understand manner, and is not necessarily limited to having all of the described configurations. Furthermore, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Moreover, it is possible to add, delete, or replace parts of the configuration of each embodiment with other configurations. [Explanation of Symbols]

[0077] 10, 20, 30, 40, 50: Optical fiber sensing system 100, 160, 170, 180, 190: Interrogator 101, 105, 161: Laser 102, 106: Laser Mount 103: Current & Temperature Controller 104: Signal Generator (SG) 110, 142: Optical switch 111: Modulators and Optical Amplifiers 120a, 120b, 120c, 120d, 120e, 201, 202: Optical couplers 121, 141: Circulator 130a, 130b: Detector 140: Reference optical path length switch 143: Optical fiber 144: Miller 150: Signal Processing Device 151: Control device 152: Frequency Mixer 153: Low-pass filter 200: Sensor unit 203, 204: Fiber Bragg Grating (FBG) 300: Fiber Optic 400: Modulated signal

Claims

1. An interrogator device that transmits measurement light to an optical fiber and analyzes the reflected light from the optical fiber, A laser light source that emits laser light, Photodetector and, Processing device and It is equipped with input / output ports, The processing apparatus controls the drive current of the laser light source to be modulated, The laser light source outputs laser light of multiple wavelengths that are different from each other, and outputs the laser light to the input / output port while continuously changing the wavelength centered on one of the different wavelengths by modulating the drive current. The photodetector detects the reflected light input from the input / output port and outputs a measurement signal. The processing device analyzes the measurement signal, discriminates the distance based on the time it takes from the output of the laser light from the input / output port until the reflected light is input back into the input / output port, and measures the reflected wavelength shift of the reflected light according to the discriminated distance. Interrogator device.

2. An interrogator device according to claim 1, Equipped with an interferometer, The interferometer outputs interference light obtained by interfering a reference light, which is a portion of the laser light output from the laser light source, with the reflected light. The photodetector detects the interference light and outputs the measurement signal. Interrogator device.

3. An interrogator device according to claim 1, Equipped with an additional signal generator, The signal generator outputs a modulation signal to cause the laser light to be output from the laser light source in response to control by the processing device. The processing unit analyzes the modulated signal and the measurement signal. Interrogator device.

4. An interrogator device according to claim 3, It also features a frequency mixer, The modulated signal is a chirp signal whose frequency changes over time. The frequency mixer outputs a beat signal obtained by mixing the modulated signal and the measurement signal. The processing unit analyzes the beat signal. Interrogator device.

5. An interrogator device according to claim 3, The modulated signal is a pseudo-random number signal. The processing device, when analyzing the modulated signal and the measured signal, determines the distance and measures the reflected wavelength shift by changing the amount of time shift of the modulated signal relative to the measured signal and calculating the correlation value between the modulated signal and the measured signal. Interrogator device.

6. An interrogator device according to claim 3, The modulated signal is a pulse signal, When the processing device analyzes the modulated signal and the measured signal, it discriminates the distance based on the delay time of the measured signal from the modulated signal and measures the reflected wavelength shift. Interrogator device.

7. An interrogator device according to claim 2, Further equipped with a reference optical path length switch, The reference optical path length switch comprises at least a plurality of reference optical fibers having different optical path lengths, and outputs the reference light by moving it back and forth through one of the plurality of reference optical fibers having an optical path length corresponding to the distance to be measured, and the interferometer outputs the interference light obtained by interfering the reference light output from the reference optical path length switch with the reflected light. Interrogator device.

8. An optical fiber sensing system comprising an interrogator, an optical fiber connected to the interrogator, and a plurality of sensor units installed at different distances from the interrogator relative to the optical fiber, The interrogator comprises a laser light source that outputs laser light, a photodetector, and a processing unit. Each of the aforementioned sensor units comprises at least two fiber Bragg gratings configured to reflect laser light of different wavelengths from each other. The processing apparatus controls the drive current of the laser light source to be modulated, The laser light source outputs laser light of different wavelengths, and outputs the laser light while continuously changing the wavelength centered on one of the different wavelengths by modulating the drive current. Each of the fiber Bragg gratings in each of the sensor units reflects the laser light according to the wavelength of the laser light that is input from the interrogator to the optical fiber, branched from the optical fiber, and incident on each of the sensor units. The photodetector detects the reflected light that is reflected by the fiber Bragg grating of any of the sensor units and input to the interrogator via the optical fiber, and outputs a measurement signal. The processing device analyzes the measurement signal, discriminates the reflected light from each of the sensor units according to the distance to each of the sensor units, and measures the reflected wavelength shift of the fiber Bragg grating that reflected the laser light at each of the sensor units. Optical fiber sensing system.

9. The optical fiber sensing system according to claim 8, The aforementioned interrogator further comprises an interferometer, The interferometer outputs interference light obtained by interfering a reference light, which is a portion of the laser light output from the laser light source, with the reflected light. The photodetector detects the interference light and outputs the measurement signal. Optical fiber sensing system.

10. The optical fiber sensing system according to claim 8, The aforementioned interrogator further comprises a signal generator, The signal generator outputs a modulation signal to cause the laser light to be output from the laser light source in response to control by the processing device. The processing unit analyzes the modulated signal and the measurement signal. Optical fiber sensing system.

11. The optical fiber sensing system according to claim 10, The aforementioned interrogator further comprises a frequency mixer, The modulated signal is a chirp signal whose frequency changes over time. The frequency mixer outputs a beat signal obtained by mixing the modulated signal and the measurement signal. The processing unit analyzes the beat signal. Optical fiber sensing system.

12. The optical fiber sensing system according to claim 10, The modulated signal is a pseudo-random number signal. The processing device, when analyzing the modulated signal and the measurement signal, calculates a correlation value between the modulated signal and the measurement signal by changing the amount of time shift of the modulated signal relative to the measurement signal, thereby discriminating the reflected light from each sensor unit according to the distance to each sensor unit. Optical fiber sensing system.

13. The optical fiber sensing system according to claim 10, The modulated signal is a pulse signal, When the processing device analyzes the modulated signal and the measurement signal, it discriminates the reflected light from each sensor unit according to the distance to each sensor unit based on the delay time of the measurement signal from the modulated signal. Optical fiber sensing system.

14. The optical fiber sensing system according to claim 9, The aforementioned interrogator further comprises a reference optical path length switch, The reference optical path length switch comprises at least a plurality of reference optical fibers having different optical path lengths, and outputs the reference light by moving it back and forth through one of the plurality of reference optical fibers, the reference optical fiber having an optical path length corresponding to the distance to the sensor unit to be measured, and the interferometer outputs the interference light obtained by interfering the reference light output from the reference optical path length switch with the reflected light. Optical fiber sensing system.

15. The optical fiber sensing system according to claim 8, Each of the fiber Bragg gratings in each of the sensor units is in a different mounting state or surface treatment state, and the reflection wavelength shift of each fiber Bragg grating changes depending on the mounting state or surface treatment state. Optical fiber sensing system.

16. The optical fiber sensing system according to claim 15, The aforementioned mounting state or surface treatment state is a state in which a sensitive film or sensitive substance made of any material or substance is attached to or in contact with, or a state in which it is held so as not to be subjected to tension. Optical fiber sensing system.

17. The optical fiber sensing system according to claim 8, Each of the aforementioned sensor units is installed in a branched optical fiber that has been branched from the optical fiber, and each of the aforementioned fiber Bragg gratings of each of the aforementioned sensor units is provided on each of the at least two measuring optical fibers that have been branched from the branched optical fiber. Optical fiber sensing system.

18. The optical fiber sensing system according to claim 8, Each of the aforementioned sensor units is installed in a branched optical fiber that has been branched from the optical fiber, and each of the aforementioned fiber Bragg gratings of each of the aforementioned sensor units is provided in series with the branched optical fiber. Optical fiber sensing system.

19. A sensing method using an optical fiber sensing system comprising an interrogator, an optical fiber connected to the interrogator, and a plurality of sensor units installed at different distances from the interrogator relative to the optical fiber, wherein the interrogator comprises at least a laser light source that outputs laser light, a photodetector, and a processing unit, and each of the sensor units comprises at least two fiber Bragg gratings configured to reflect the laser light of different wavelengths, The processing device controls the drive current of the laser light source to be modulated, By modulating the drive current, the laser light source is output while continuously changing the wavelength, centering on one of the two different wavelengths. The laser light is input from the interrogator into the optical fiber, branched from the optical fiber, and incident on each of the sensor parts, and according to the wavelength of the laser light, the laser light is reflected by one of the fiber Bragg gratings. The photodetector detects the reflected light that has been reflected by the fiber Bragg grating of any of the sensor units and input to the interrogator via the optical fiber, and outputs a measurement signal. The processing device analyzes the measurement signal, discriminates the reflected light from each of the sensor units according to the distance to each of the sensor units, and measures the reflected wavelength shift of the fiber Bragg grating that reflected the laser light at each of the sensor units. Sensing method.

20. A sensing method according to claim 19, The interferometer further provided by the interrogator outputs interference light obtained by interfering a reference light, which is a portion of the laser light output from the laser light source, with the reflected light. The photodetector detects the interference light and outputs the measurement signal. Sensing method.

21. A sensing method according to claim 19, The signal generator further provided by the interrogator outputs a modulation signal for outputting the laser light, The processing unit analyzes the modulated signal and the measurement signal. Sensing method.

22. A sensing method according to claim 20, The interrogator further includes a reference optical path length switcher, which causes the reference light to be output by reciprocating through a reference optical fiber with a path length corresponding to the distance to the sensor unit to be measured, among a plurality of reference optical fibers with different optical path lengths. The interferometer outputs the interference light obtained by interfering the reference light output from the reference optical path length switch with the reflected light. Sensing method.

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